Conventional nuclear magnetic resonance (NMR) tools and conventional resistivity tools can be used in a variety of applications from downhole logging to analysis of samples in a lab, in refineries, for food and chemical processing, and the like. These conventional tools often make use of a resonant circuit to cause an excitation signal necessary to perform the appropriate measurements. The use of resonant circuits can cause certain difficulties and restrictions in conventional resistivity and NMR tools alike.
For example, today's common approach to the excitation and detection system for resistivity logging is to employ a tuned resistor-inductor-capacitor (RLC) circuit for both excitation and detection. When logging at a multitude of frequencies is required, as it is in most cases, then the circuits have to be tuned to have peaks at multiple frequencies. The multi-tuning process is labor-intensive, and further it is known that the tuning changes with temperature and may drift to the point that it is hurtful to the measurement. When the number of resonant frequencies is large (generally three or above), then a selector switch can be used to select between several circuits with different tunings. In some cases, this selector switch itself becomes a weak point in the design.
In such a circuit, each resonant peak in frequency is completely characterized by three parameters: the resonance frequency, the impedance on resonance, and the quality factor. For a circuit with a reasonable quality factor (˜100) the resonance frequency acts to efficiently couple the energy contained in the coil during excitation to the formation and to reject noise at frequencies outside of the resonance bandwidth during detection. Both of these features are essential elements to maintaining system performance at high resonance frequencies. At low resonance frequencies there is another approach to achieving the same efficiencies based on switching technologies.
Various challenges exist with respect to the use of tuned circuits in well logging. For example, there is variation of the magnetic field with temperature, mud loading, and formation loading. The Q of the circuit is a measure of the ratio of the stored to dissipated energy in the circuit. When the temperature of the coil changes (as with formation depth) the Johnson noise in the circuit changes and the Q changes in response. When the resistivity of the material in the field of the tool changes (either from variations in mud or formation fluids) the Q changes in response. These changes lead to miss-settings of the resistivity parameters and variations in the observed signal intensity and derived relaxation times. Left uncompensated, changes in Q degrade instrument performance.
There are also limitations in the number of frequencies for which a given tuned circuit can be tuned to. In some cases this necessitates the use of several tuned circuits, each tuned to a possibly different set of frequencies.
In the situation where the resonance frequency is low (<1 MHz), tools with reasonable Qs ring. The tuned circuit acts as a bell and having driven the circuit with a few hundred to a thousand volts, waiting is necessary until the natural response decays to of order tens of micro-volts before acquiring a signal. Additionally, it is appealing to run the logging tool at multiple-depths to help characterize the formation. With the resonant approach changing depth is accomplished by changing the resonance frequency, which demands a change in the tuning and in the electronics.
Finally, the characteristic of the formation may require a different setting to be used (for example in terms of frequency of excitation signal). Pre job modeling and planning attempts to select the best settings for the LWD tool, but changing formation characteristics can present a significant uncertainty. Also, for complex formations it may be desirable to be able to change the settings of the LWD tool from one time to another. Hence, flexible hardware is highly desirable.
Similar issues arise with respect to conventional downhole NMR tools. Conventional NMR tools commonly use metallic wire wound coils to excite nuclear spins by passing an RF current and generating an RF magnetic field, and to detect the nuclear spin magnetization by receiving the electric current induced in the coil. In order to achieve efficient coupling of this coil and the rest of the electronics, this coil is often connected with a capacitor in parallel to form a resonance circuit. Such a parallel circuit is known to be resonant at a frequency f0:
      f    0    =      1          2      ⁢                        π          ⁡                      (            LC            )                                                1            /            2                    ⁢                                                    where L is the inductance of the coil and C is a capacitance of the capacitor. This frequency f0 is called the resonance frequency.
For conventional NMR circuits, this resonance frequency is adjusted by the choice of the capacitor to match the value of the Larmor frequency of spin precession for a given magnetic field. For example, at a magnetic field of 1 Tesla, the Larmor frequency of a hydrogen atom is 42.58 MHz. A second capacitor is often used to adjust the impedance of this resonance circuit to match the impedance (e.g. 50 ohm) of the power amplifier and receiver. It is a common practice that all subsystems (coil system, receiver, and power amplifiers, and cables) are selected or tuned to be 50 ohm, even though other values of the impedance have been used. Although the exact value of the impedance can be a matter of choice, the impedance matching is important for efficient power transmission to the coil to achieve good RF B1 field and also optimal receiver sensitivity.
In practice, variable capacitors are often used to adjust the resonance frequency (tuning) and the impedance (matching). Both tuning and matching conditions can vary during an experiment due to component drift, changes in temperature and sample properties. An NMR experimentalist needs to adjust frequently either capacitors to maintain a good matching and tuning condition. A poorly adjusted tuning and matching condition can lead to deterioration of signal, poor control of spin dynamics and unwanted signals.
One embodiment disclosed herein is a novel method to couple the coil to the NMR electronics without the need to tune the coil system to a resonant frequency at the Larmor frequency. The coil may be connected to the RF electronics directly or through electronics switches and the response of such a system can be much more broadband than a conventional resonance coil system. The main benefit of such system is that there may be no need for the tuning and matching capacitors and no need to adjust the circuit during NMR experiments and for different samples. This method is particularly useful for NMR at low frequency or when the coil inductance is very low, so that the impedance of the coil at the Larmor frequency (2pi*f0*L) is small.
For NMR applications in oilfield exploration such as NMR logging and LWD NMR logging, this new approach can eliminate the need to adjust the tuning capacitors and the associated relays and other electronics and greatly improve the robustness of these logging systems. The system will also be more stable under different conditions, such as different sample or environmental salinity and different fluids.
The science and technology of NMR involves two main aspects. One is the science of the nuclear spin system and the use of magnetic fields to control their behavior and dynamics. The essential phenomenon of NMR is the resonant absorption and irradiation of RF energy by the nuclear spins in a static magnetic field, B0. The frequency of the irradiation is called Larmor frequency, f=gamma*B0, where gamma is the gyromagnetic ratio specific to each element. For example, a commonly used nucleus for NMR is proton with a gamma=4258 Hz/G. The applied magnetic fields are often of two kinds. One is the static field that determines the Larmor frequency. The second is RF irradiation at this Larmor frequency. It is used to perform spin rotation.
The second part of the NMR is the associated electronics to (1) apply the RF irradiation at the Larmor frequency to the spin system and samples in order to control the behavior and dynamics of the spins; and (2) to receive the spin precession signals. One design goal of such electronics is the efficient transmission of RF power to the sample region in order to achieve control of the spin system. The tuning circuit is well suited for such power transmission. For example, assuming the power amplifier and the circuit are tuned to a common impedance of 50 ohm, then the maximum of the power from the amplifier will be delivered to the circuit. Since the capacitors are often of higher quality factor than the coil, the applied power is dissipated in the coil resistance to produce the maximum electrical current and in turn the maximum RF B1 magnetic field. However, if the impedance of the coil system is significantly different from that of the amplifier, less power will be deposited in the coil and thus less optimal B1 field.
In addition to the efficient transmission of the RF power and efficient signal reception from the matched impedance, the resonance circuit also behaves as a band-pass filter centered at the resonance frequency. The width of the bandwidth is controlled by the resistance of the circuit, often dominated by that of the coil. The ratio of the resonance frequency and the width of the resonance is often called the Q factor. For common NMR circuits, Q is often on the order of 100. Only the signals within the bandwidth of the circuit are effectively coupled to the output electronics. Signals outside the bandwidth are attenuated. The tuned coil acts as a narrow band filter.
To summarize, in a conventional NMR system, there are two resonance phenomena. One is the resonant absorption of the spin system of the RF energy and the resonant manipulation of the spin system by the RF pulses. The frequency of this resonance is determined by the externally applied magnetic field and the choice of the nuclei to be interrogated. The second resonance is the electronics and circuit of the NMR equipment to supply the RF pulses and to detect NMR signal. The conventional NMR circuit using resonance circuit can be an excellent circuit to achieve efficient transmission and reception. Certain embodiments disclosed herein include an alternative method for the NMR circuit that does not require a resonant condition. This will enable a more broadband transmission of RF pulses and reception of NMR signals.
Thus, needs in the art exist for systems and methods that address some of the deficiencies in conventional tools, such as some of the deficiencies described above.